I built my computer around 4-5 months ago and was adding more RAM then when I started it up the CPU LED was staying lit. So we tried changing out the different sticks of RAM and it still wouldn't go past. We then tried the MemOK button but to no avail. So we took out the CPU and looked at it to make sure nothing was bent and when we put it back in the computer now just turns on for half a second then shuts back down. Fans and lights come on (Including the CPU fan).
ASUS P8P67 Motherboard
Intel i5 2500k Quad Core with 3.3 GHz
8 GB DDR3 RAM (Before) 16 GB (Total)
Corsair TX 750w PSU
AMD HD Radeon 6950
I had the exact same problem twice, once it was the PSU (one of the fan connectors was shorted which caused the PSU to shut down at once for safety reasons, which btw are many). Make sure all the connections, HDD, SDD, fans...etc are wired properly. what's your PSU?
2nd time was a bent pin on the MB.